Attaboy Tom. We need more hams who build their own antennas and for that
matter other ham equipment. The most important thing you can do to improve
your antennas is install a choke on the coax where it connects to the DE.
Here's why. The RF travels up the inside of your coax to the attachment
point. At the point where the shield connects to the DE the current on the
inside of the shield splits with part of it going out to the DE and some of
it going right down the outside of your coax and radiating as a vertical
antenna. This will cause some loss of gain, decrease in the side null depth
and noise pickup. It is very easy to improve this. You can build or buy a
sleeve balun with many ferrite cores around the outside of the coax. You can
even coil up a bit of the coax around a piece of PVC pipe to make a coil at
the attachment point. If you want more information you can go to our clubs
web page www.sdxa.org and look under DX tips where there is a good amount of
information on various types of baluns you can build. You can get snap on
ferrite sleeves for about a buck apiece from All Electronics with a 1/2 inch
opening. All ferrites are not the same but I have made a few measurements on
those and they are reasonably good for what you are doing. I would put 20 or
so on the coax. The subject of ferrite baluns is more complex than what I
have told you. You can read the information by K9YC which goes into the
subject in more depth or you can buy a balun from DXEngineering which
incorporates the latest materials to make a great wide band balun but they
are a bit costly.
Re the element spacing and tuning vs performance, I have been using the
info on the CD that comes with the 4th edition of ON4UN's Low Band Dxing
Handbook . He has over a hundred different Yagi designs there. You can
select 2 element type, various spacings, Director or reflector, and he gives
you the F/B, gain, feed point impedance, and element length for each. The
lengths are for a uniform diameter element. You can then use a program
called Taper and enter your element diameter and length of each section of
it and get the exact length needed. I am too lazy to use the usual modeling
programs with all the stuff you must enter. This works for me. There is
information on gamma match and omega match design. I kind of like the idea
of using a ferrite core transformer to step up the 25 ohm or so impedance
of the antenna to 50 ohms. That is how SteppIR handles matching their
antennas with a single xfmr covering all the bands.
73, Dan, N5AR
[mailto:firstname.lastname@example.org]On Behalf Of Tom Osborne
Sent: Friday, May 02, 2008 2:34 PM
Subject: [TowerTalk] 1/4 wave spacing on 2 el beam
I built a couple of 2 el beams, one for 20 and one for 10.
I just split the driven element, fed them directly with 50 ohm coax. and
spaced the reflector at a 1/4 wave. It seems to be a good match on both
antennas and the SWR is very low.
I don't have any modeling software here, so what does it say about an
antenna like that? I know the best gain is with a director a lot closer,
but this way I don't need to mess with a gamma or any other kind of match.
I like the KISS method of doing things. 73
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